fix: remove deprecations

[novotnicek]

I getting some deprecation (PHP 8.4.5):

Implicitly marking parameter $foobar as nullable is deprecated, the explicit nullable type must be used instead

Please check it when you have time.

Thanks!

[williarin]

Thanks @novotnicek, could you also change ubuntu-20.04 to ubuntu-latest in the workflow file? The tests don't run because 20.04 has been removed last month.

[novotnicek]

I dunno what's wrong, but CI pipeline failing on composer. ¯\_(ツ)_/¯